Tracing HIV's Steps

DENVER, COLORADO--Clarifying the origin of AIDS won't prevent or cure any HIV infections, but the mystery has long gripped the field. Two groups studying wild chimpanzees in Cameroon reported progress on that front here this week at the 13th Conference on Retroviruses and Opportunistic Infections.

These teams earlier had found persuasive evidence that chimps harbor a simian immunodeficiency virus called SIVcpz that became HIV-1--the predominant cause of AIDS in humans (ScienceNOW, 1 February 1999,)--but they had discovered precious few infected animals to make the case. Their next step was to analyze 1300 fecal samples of wild apes (some turned out to be from gorillas) and identify several places in which SIVcpz was widespread in chimp communities. Then they genetically characterized dozens of new isolates.

The new work found more than 30 strains of SIVcpz, tripling the number previously discovered. The researchers took advantage of the fact that chimps cannot swim, which means that rivers block the spread of viruses. For the first time, they found chimp communities in which SIVcpz infection was widespread--in one, up to 35% of the animals analyzed had the virus in their feces. "Our eyeballs popped out of our heads," said Brandon Keele, who works with Beatrice Hahn at the University of Alabama, Birmingham.

In a separate presentation, Fran Van Heuverswyn, part of a team headed by Martine Peeters of the Institut de Recherche pour le Dévelopement in Montpellier, France, described how two of the isolates more closely matched the HIV-1 causing the human epidemic than any found in the past. (HIV-2, which infects humans much more infrequently, clearly comes from an SIV in sooty mangabeys.)

Building on the new data, Paul Sharp, who studies molecular evolution of pathogens at Nottingham University, United Kingdom, explored the origin of SIVcpz. Sharp's new analysis suggests that the SIVcpz closest to HIV-1 is a combination of SIVs isolated from red cap mangabeys and monkeys from the Cercopithecus genus.

Putting in the final piece of the puzzle, Sharp said the virus must have reached a major city to start the AIDS epidemic. He posited that a person became infected in rural Cameroon and then traveled by river to what is now known as Kinshasa, Democratic Republic of Congo. Kinshasa has the greatest genetic diversity of HIV-1, suggesting that the virus has been there longer than anywhere else. It also was home to the first known HIV-infected person, a Bantu man who had his blood sampled in 1959 for a malaria study.

"They're closing in on some very hot stuff," said James Hoxie, who studies HIV and SIV at the University of Pennsylvania, Philadelphia. "It's compelling genetic evidence."